Circuit breaker monitor for uninterruptable power systems including a static bypass

ABSTRACT

An uninterruptable power system including main apparatus for supplying power to a critical bus, the main apparatus includes electromechanical circuit breakers and a static switch. Bypass apparatus is also provided including a static switch to supply power to the critical bus in high-speed response to the actuation of the circuit breaker trip initiating means.

United States Patent 91 Williamson 541 CIRCUIT BREAKER MONITOR FOR Jan.30, 1973 [5 6] References Cited UNITED STATES PATENTS 3,530,360 9/1970Relation ..317/26 2,785,317 3/1957 Langberg ..,.307/68 X 3,201,5928/1965 Reinert .307/68 X Primary Examiner-D. F. Duggan AssistantExaminer-Harvey Fendelman Attorney-J. Wesley Haubner et a1.

[57] ABSTRACT An uninterruptable power system including main apparatusfor supplying power to a critical bus, the main apparatus includeselectromechanical circuit breakers and a static switch. Bypass apparatusis also provided including a static switch to supply power to thecritical bus in high-speed response to the actuation of the circuitbreaker trip initiating means.

5 Claims, 2 Drawing Figures 5Q-ZI9 G ur/ury M Q MAM 6 A 67/1776 g 8 M aSWITCH J' M /7 M Y 4 3 V g 0- RECT/F/EI? A 11v V1.7: 76R 57A c STA/V08)1 sw/rcH E GENERATOR g l 1 i1 U v 1 Pas: g 5 1 69 g I Z MON/TOR Q E l lI I I l c r am 77c RIP I I AND TRANSFER l- I 48 CIRCUIT I 7 1 t 1C/ACU/T Z2 C C B/PEA KER MEA KER m/P TR IP MON/70R TRIP //v/r/,4 r/n/ellwr/A r/m; MEANS MIA/VJ cmcu/r anew/r0? TRIP CIM/TROL cmcu/r 24 if 2a)7 I I I 1 1 e. 1 a a FAN CpOlfi/VUL nvvznrsk 1 2/6 "2A6 Momma/N6 MOMMWGER CHARGING I emu/llama a A y REL AY MONITOR/N6 REY/870A curour ITRAMSFO/Mdffi RELAY MONITOR/N6 RELAY gamer/Juana PATENTEDJAH 30 1925SHEET 101- 2 QN G PATENTEDJAN 30 975 SHEET 2 OF 2 CIRCUIT BREAKERMONITOR FOR UNINTERRUPTABLE POWER SYSTEMS INCLUDING A STATIC BYPASSBACKGROUND AND OBJECTS OF THE INVENTION -technology: US. Pat. Nos.3,201,592-Reinert et al.,

3,309,570-Goldberg, 3,530,360-Relation.

In the art of electric power distribution and protection, it is becomingincreasingly important to minimize adverse effects of an abnormalcondition in the source of power, in the distribution system itself, orin the electric load supplied thereby so that critical loads cancontinue operating with the least possible disturbance. Reliable powercan be essential for loads such as computers, for example, where even atransient dip in voltage, or momentary loss of power, can result inserious errors or malfunctions in the utilization equipment and anextended outage could be intolerable. Consequently, to improve thequality and continuity of electricity supplied to such loads, electricalmanufacturers have made available uninterruptable power systems,sometimes referred to as no-break power supplies, for installationbetween the incoming power lines and the load. Such apparatus typicallycomprises combinations of rectifying, energy-storing, and invertingmeans so arranged as to faithfully energize a critical bus with highlystable A C power regardless of disturbances in or failure of utilitypower.

In one common type of uninterruptable power system, which may be calleda redundant inverter system, the critical bus is supplied by a pluralityof inverters operating in parallel. In US. Pat. No. 3,530,360-Relation,which is assigned to the same assignee as my invention, there isdisclosed and claimed means for detecting an impending failure of one ofthe inverters in such a system and for rapidly isolating it from thecritical bus so that the faulty inverter itself does not disturb theload which continues to be energized by the other, sound inverters.

Another type of uninterruptable power system, which may be called anon-redundant, single-inverter system, consists of a single inverter forsupplying power to the critical bus under normal conditions. The systemalso includes a bypass circuit for supplying power to the critical busin the event that the inverter malfunctions. To that end a first staticswitch may be provided between the inverter and the bus and,asecondrstatic switch may be provided in the by-pass circuit, the firstswitch to be rendered non-conductive and the second switch to berendered conductive in response to the detection of an inverter fault.As soon as the malfunction is cured, this switching process is reversedand the inverter is returned to service, thereby restoring the desiredbuffer between the incoming power lines and the critical load.

Irrespective of the type of uninterruptable power system (i.e., whethera redundant inverter system or a 3 ,337,742-Baeher et al.,

non-redundant system) it is a desirable practice to include anelectromechanical circuit breaker on the D-C input side of the inverterand an electromechanical circuit breaker on the A-C output side thereofin order to completely isolate the inverter whenever it is out ofservice for maintenance or repair purposes. Such breakers are ordinarilyarranged to be automatically tripped in the event of certain failuremodes in the inverter or its auxiliaries (e.g., commutation failure,failure of the cooling fans, etc.). In a non-redundant single invertersystem it is also important to switch or transfer to the bypass circuitin response to the detection of any such malfunction so that power fromthe utility mains is provided to the critical bus without interruption.

A general object of my invention is the provision, for a non-redundantsingle inverter uninterruptable power system, of improved means forinitiating high-speed transfer to a bypass circuit in response to thedetection of any number of circuit abnormalities without a complexnumber of interlocks and sensors.

SUMMARY OF THE INVENTION In carrying out my invention in one form Iprovide an uninterruptable power system including an inverter forconverting D-C power to A-C power and supplying same to a critical bus.The input of the inverter is through a first electromechanical circuitbreaker and the output of the inverter is through a secondelectromechanical circuit breaker. Each circuit breaker includes tripinitiating means which is operative when actuated for initiating theopening of the circuit breaker. The trip initiating means are actuatedby system monitoring means in response to certain abnormal conditionsthat may sometimes develop in the inverter or other parts of theapparatus. A first normally conductive static switch is connectedbetween the inverter and the critical bus and a second normallynonconductive static switch is connected in a bypass circuit betweenutility mains and the critical bus.

Means are provided to monitor the circuit breakers trip initiating meansand to render the first static switch nonconductive and the secondstatic switch conductive in high speed response to the actuation of suchmeans.

DESCRIPTION OF THE DRAWINGS My invention will be better under and itsvarious objects and advantages will be more fully appreciated from thefollowing description taken in conjunction with the accompanyingdrawings in which: FIG. 1 is a one-line schematic diagram of anuninterruptable power system embodying my invention.

FIG. 2 is a schematic diagram of a portion of the uninterruptable powersystem shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG.1, which is a one-line schematic diagram of a three-phaseuninterruptable power system in accordance with my invention, a criticalA-C electric power bus 1 is seen connected to a D-C bus 2 by mainconducting apparatus 3 comprising the series combination of a D-Ccircuit breaker 4, an electric power inverter 5, a thyristor-composed,forced-commutation static switch 6, and an AC circuit breaker 7.

The main apparatus, under normal conditions, is adapted for supplyingA-C power to the critical bus. The critical bus is also adapted fordirect energization from electric utility mains 8 in the event thatthere is an electrical abnormality in the main apparatus. To that endbypass apparatus 9 is connected between the utility mains and thecritical bus 1. The bypass apparatus includes a thyristor-composedstatic switch 1 1.

The D-C bus 2 is supplied by a rectifier 12 that is normally energizedby power delivered to an A-C bus from the electric utility mains 8.Reliable energy storing means such as a storage battery 13, which iskept charged by the rectifier 12, is provided to energize the D-C busduring shortterm outages of the mains. An emergency generating meanssuch as a diesel standby generator 14 is provided to supply power, viacircuit breaker 15, to the A-C bus 10 in the event of a longterm loss ofutility power from the mains 8.

The critical A-C bus 1 is used to supply highly reliable, stable powerto critical A-C load circuits. The load typically comprises a pluralityof feeders or branches (not shown) each being connected to bus 1 andbeing individually protected by suitable overload responsive means suchas electric fuses. The feeders may be used to supply power to one ormore computers and peripheral equipment therefor or to other electricalapparatus requiring stable A-C power.

In normal operation of the uninterruptable power system shown in FIG. 1,the A-C bus 10 is energized via a closed circuit breaker 16 from theelectric utility mains 8. The rectifier 12 is energized from the A-C busvia a closed circuit breaker 17. The output of the rectifier serves toenergize the D-C bus 2 via a closed circuit breaker 18 as well as tocharge the battery 13 via a closed circuit breaker 19.

The main apparatus 3 is operative for converting the D-C power from thebus 2 and for supplying it to the critical A-C bus 1. To that end theD-C power from the D-C bus is utilized to energize the inverter 5 viacircuit breaker 4 which is in its conductive state. The inverteroperates in synchronism with the A-C power from the electric utilitymains 8 so that its alternating voltage output is in phase with thevoltage on bus 10. (In my U.S. Pat. No. 3,6l4,46l there is shown andclaimed means for accomplishing such a synchronization). The sinusoidalAC power output of the inverter is fed via the static switch 6, which isin its conductive state, and the circuit breaker 7, which is also in itsconductive state, to the critical bus. Both of the inverters input andoutput circuit breakers 4 and 7 are of the electromechanical type so asto insure that the inverter is safely isolated from the D-C bus and thecritical A-C bus respectively, when such isolation is desired.

Although many types of electromechanical circuit breakers may beutilized, in one installation of my invention circuit breaker 4 is amolded case circuit breaker, type G.E.-TKMA836, and circuit breaker 7 isa molded case circuit breaker, type TKM836Y. Each of these circuitbreakers is equipped with a conventional trip initiating means (e.g., aG.E.-TKMAUV11 undervoltage release) which is operative when actuated forinitiating the tripping cycle of the circuit breaker. For example,circuit breaker 4 is equipped with trip initiating means 4A and circuitbreaker 7 is equipped with trip initiating means 7A. Each of the tripinitiating means includes a solenoid composed of a trip initiating coiland a plunger, the solenoids plunger is biased by a spring and iscoupled through mechanical linkages to a tripping mechanism (e.g.,button) in the circuit breaker. As long as the coil of the initiatingmeans is energized (i.e., the voltage thereon is above a predeterminedlevel) the plunger will be held within the coil and against the actionof the spring. Upon the deenergization of the coil the spring forces theplunger through its associated linkages into engagement with thetripping mechanism to initiate the opening of the circuit breakers maincontacts. The trip initiating coil of trip initiating means 4A isdenoted as UV4 and is shown in phantom lines in FIG. 2, and the tripinitiating coil of trip initiating means 7A and denoted as UV7 is alsoshown in phantom lines therein. The other abovedescribed components ineach of the trip initiating means are not shown in the drawing.

It should be appreciated that other trip initiating means than the abovenoted undervoltage release can be utilized. As will soon be explainedmore fully, plural monitoring means or relays are provided to actuatethe trip initiating means of the circuit breakers 4 and 7 in the eventthat an abnormality is detected in the apparatus.

During normal operation of the uninterruptable power system, i.e., whenthe main apparatus is supplying power to the critical bus, the staticswitch in the bypass apparatus is in its nonconducting state whereby itblocks the flow of A-C power directly between the utility mains 8 andthe critical bus 1.

In the event of a fault in the inverter (e.g., a commutation failure) itis of utmost importance to quickly isolate the inverter from thecritical bus and at the same time render the static switch in the bypassapparatus conductive so that the power supply to the critical bus willnot be interrupted. To accomplish that end a fuse monitoring circuit 20,like that disclosed in the abovenoted Relation patent, which can bereferred to for further details of construction and operation, isconnected to the inverter 5. This circuit is adapted for monitoring theinverter fuses and for causing a switch controlling circuit 21,hereinafter called a static trip and transfer circuit, to turn off orcommutate, the conducting thyristors in the static switch 6 within a fewmicroseconds of the detection of a fault in the inverter. In response tothis same event and in the interests of safety the opening of theelectromechanical output circuit breaker 7 is initiated. The staticstrip and transfer circuit also serves to render the thyristors in thebypass static switch 11 conductive within a very short time after thedetection of the fault. Once static switch 11 begins conducting, poweris supplied to the critical bus from the utility mains notwithstandingthe isolation of the inverter from the bus. The transfer of power fromthe main apparatus static switch to the bypass apparatus static switchoccurs so fast that the flow of power to the critical bus is virtuallyuninterrupted.

In the case of certain apparatus abnormalities in addition to inverterfailures, it is also of considerable importance to terminate powerconduction through the main apparatus and to commence power conductionthrough the bypass apparatus before any disturbance of the critical busvoltage can result. To that end I provide circuitry for rendering thebypass static switch 11 condu ctive (this function being hereinafterreferred to as the transfer function) and for terminating conduction inthe main static switch 6 (this function being hereinafter referred to asthe trip function) in high-- speed response to the detection of a systemabnormality by any monitoring relay coupled to the trip initiating meansfor the inverters input or output circuit breakers 4 and 7.

in accordance with my invention the trip and transfer functions areaccomplished in a relatively simple manner which does not necessitatecomplex circuitry or interlocks between the relays monitoring the systemconditions and the circuitry controlling the conduction status of thebypass apparatus and the main apparatus static switches. Furthermore,the trip and transfer functions are accomplished extremely quickly, evenbefore the electromechanical circuit breakers can open. In particular Iprovide means which monitors the trip initiating means in the inverterinput and output circuit breakers and provides a signal to initiate thetrip and transfer functions in high-speed response to the actuation ofthe circuit breakers trip initiating means. Such means is shown in FIG.1 and includes a trip monitor circuit 22 which monitors the voltage onthe trip initiating coils UV4 and UV7 of the circuit breakers 4 and 7.

The trip initiating coils are connected to trip control circuit 23 forthe circuit breakers 4 and 7. This circuit is shown in detail in FIG. 2and serves to keepthe trip initiating coils energized when apparatusconditions are normal. As can be seen the circuit is connected acrossthe D-C bus (i.e., between the relatively positive D-C conductor andground) via a protective fuse F1 and a current limiting resistor R1 inseries therewith. The circuit comprises a plurality of relay contacts,namely, C1, C2, C3 and C4 connected in series circuit relation with oneanother. The series combination of contacts C1, C2, C3 and C4 isconnected in series with the series combination of another pair of relaycontacts C5, a resistor R2 and a freewheeling diode D1. The undervoltagecoil UV4 of trip initiating means 4A is connected across the diode D1.The series combination of contact C1 through C4 is also connected inseries with the series combination of another set of contacts, C6, aresistor R3 and another freewheeling diode D2. The undervoltage coil UV7of trip initiating means 7A is connected across the diode D2.

Each of the circuit breaker trip control circuit contacts is controlledby a device, (e.g., a relay) which monitors a specific apparatuscondition. For example, the D-C bus voltage is monitored by a D-Cmonitoring relay 24. This relay is arranged to keep contact C1 closed aslong as the D-C bus voltage is above a preselected level. The operationof a fan for cooling the apparatus is monitored by a fan monitoringrelay 25. This relay is arranged to keep contact C2 closed as long asthe fan is operating properly. A relay, denoted as the control powermonitoring relay 26, monitors the existence of control power for theapparatus and keeps contact C3 closed so long as such control powerexists. A relay 27 is provided to determine whether the resistor (notshown) which is utilized for initially charging the converterscommutation capacitors (not shown) has been cut out of the power circuitand for keeping contact C4 closed if such has occurred. A relay, 28, isprovided to determine if a circuit breaker (not shown), connecting azig-zag transformer (not shown) in the power circuit to provide aground-toneutral conductor for the critical bus, is closed, and forkeeping contacts C5 and C6 closed if such is the case.

As should be appreciated, if all of the above-noted monitored systemconditions are normal, contacts C1-C6 will be closed and theundervoltage release coils UV4 and UV7 will be energized therethroughfrom the D-C bus. In particular, the undervoltage coil UV4 will beenergized by the current flowing from the D-C bus through fuse F1, thecurrent limiting resistor R1, the closed contacts C 1, C2, C3, C4 andC5, and the resistor R2. In a similar manner the undervoltage coil UV7will be energized by the current flowing from the D-C bus through thefuse F1, the resistor R1, the closed contacts C1, C2, C3, C4 and C6, andthe resistor R3.

As previously noted the undervoltage coils UV4 and UV7 are arranged suchthat upon being actuated by deenergization (i.e., when the voltagethereacross drops below a preselected level) they set in motion thetripping operation in their respective electromechanical circuitbreakers 4 and 7. At the same time, the drop in the voltage across thecoils is detected by the trip monitor circuit 22 and a signal indicativethereof is provided to the static trip and transfer circuit 21. Inresponse to that signal the static trip and transfer circuit provides atripping signal to the main apparatus static switch and at the same timeprovides a closing signal to the bypass apparatus static switch. Inresponse to the tripping signal, commutation means (not shown) in themain static switch are activated to render the static switchnonconductive. This action occurs within a very short period of time(e.g., 50 microseconds or less); in response to the provision of theclose signal for the bypass apparatus static switch, the thyristorstherein begin conducting power to the critical bus from the A-C bus 10.The starting time (i.e., the time which elapses before the static switchconducts in response to the close signal) is quite small (e.g., 25microseconds).

Accordingly, both the trip and transfer functions will be effectuatedwell before the opening of the electromechanical circuit breakers 4 and7. Furthermore, owing to the short turnoff time of the main apparatusstatic switch and the short starting time of the bypass apparatus staticswitch, the duration of the switching transient from the time a trip andtransfer signal is initiated is of the order of 1 millisecond or less,including settling'time, whereupon the voltage on the critical bus willremain virtually undisturbed.

The details of a preferred embodiment of the trip monitor circuit 22 areshown in FIG. 2. As can be seen therein the inputs to the circuit areprovided via a pair of diodes D3 and D4 connected to form an or gate.The or gate inputs are connected across the undervoltage coils UV4 andUV7, respectively. The anodes of the input diodes are connected to abiasing resistor as R4 and to the cathode of a zener diode 2D]. Theanode of the zener diode is connected to the base of a transistor Q1.The collector of the transistor is biased from a positive D-C supply viaa biasing resistor R5. The emitter of the transistor is connected toground. The output of the transistor is provided from the collector viaa pair of voltage dropping diodes D5 and D6 to the static trip andtransfer circuit 21.

Operation of the monitor circuit is as follows:

When the voltage on either coil UV4 and UV7 is above the breakdownvoltage of the zener diode, transistor 01 is biased on" via resistor R4and zener diode ZD] (the zener diode insures that the base voltage doesnot go too high with respect to the emitter voltage). When transistor Q1is on (i.e., conducting), the voltage appearing at its emitter iseffectively ground. If the voltage on either of the undervoltage coilsdrops below the breakdown voltage of the zener diode the transistor willturn off whereby a positive signal will appear at its collector. Thispositive or output signal is supplied to the static trip and transfercircuit to initiate the trip and transfer function.

It should be appreciated that the trip monitor output signal will beproduced well in advance of the opening of the circuit breakers 4 and 7notwithstanding the fact that the trip initiating means of the circuitbreakers 4 and 7 is concurrently responding to the same condition,

' i.e., the dropping of the voltage across the undervoltage coils.

Since the trip monitor circuit responds to a drop in voltage across thecircuit breaker undervoltage coils, a failure in either one of thesecoils '(which usually manifests itself in a short circuit) will alsoresult in the initiation of the trip and transfer function in the samemanner as if a monitoring relay had sensed a system abnormality. Forthis reason I prefer to monitor both UV coils rather than just one.

As can be seen in FIG. 1 a circuit breaker 29 is connected in shunt withthe bypass static switch 11. This breaker is preferably of theelectromechanical type and is closed a short period of time, (e.g. a fewcycles) after the bypass static switch is rendered conductive. Backingup a first-on static switch with a slower closing main breaker is an oldexpedient in the art (See US. Pat. Nos. 3,237,030 and 3,40l ,303). Byutilizing such a back-up circuit breaker the bypass static switch can beconstructed of smaller thyristors since they will only be required tocarry power to the critical bus for a few cycles until the circuitbreaker 29 closes.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from myinvention in its broader aspects; and I, therefore, intend herein tocover all such changes and modifications as fall within the true spiritand scope of my invention.

What i claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. in an electrical system including main apparatus and bypass apparatusfor providing A-C power to a critical bus, said bypass apparatusincluding a first normally nonconductive static switch connected betweena supply of A-C power and said bus, said main apparatus including aninverter for converting DC power into A-C power, a first normallyconductive circuit breaker connected between the inverter and a supplyof D-C power and means comprising, a second normally conductive staticswitch in series with a second normally conductive circuit breaker forconnecting the inverter to said bus, said circuit breakers includingtrip initiating means operative when actuated for causing the circuitbreakers to become nonconductive, said initiating means being actuatedin response to the detection of an abnormality in said main apparatus,the improvement comprising: swltch con rolling means operative inhigh-speed response to the actuation of at least one of said tripinitiating means for causing said second static switch to interrupt theflow of power through the main apparatus to said critical bus, and atthe same time for causing said first static switch to begin conductingpower from said A-C power supply to said bus, whereupon the flow of A-Cpower to said bus is virtually uninterrupted.

2. The electrical system as specified in claim 1 wherein said circuitbreaker trip initiating means includes undervoltage sensitive meanswhich are normally energized to a voltage above a predeterminedthreshold level in response to relay means monitoring predeterminedconditions in said main apparatus, and wherein upon the detection of anabnormality by said relay means the voltage appearing on theundervoltage sensitive devices falls below said threshold level.

3. The electrical system as specified in claim 2 wherein said staticswitch controlling means comprises means for monitoring the voltageappearing on the circuit breaker undervoltage sensitive means and forcausing said second static switch to become nonconductive and said firststatic switch to become conductive in high-speed response to the voltageon either of the undervoltage sensitive means dropping below thethreshold level.

4. The electrical system as specified in claim 3 wherein said mainapparatus also includes means for monitoring fuses in the inverter andwherein said static switch conduction controlling means includes meansfor causing said second static switch to become nonconductive and saidfirst static switch to become conductive in high-speed response to saidlast mentioned means.

5. The electrical system as specified in claim 3 wherein a third circuitbreaker is connected in shunt with said first static switch and which isrendered conductive a short time after said first static switch isrendered conductive and wherein all of said circuit breakers are of theelectromechanical type.

1. In an electrical system including main apparatus and bypass apparatusfor providing A-C power to a critical bus, said bypass apparatusincluding a first normally nonconductive static switch connected betweena supply of A-C power and said bus, said main apparatus including aninverter for converting D-C power into A-C power, a first normallyconductive circuit breaker connected between the inverter and a supplyof D-C power and means comprising, a second normally conductive staticswitch in series with a second normally conductive circuit breaker forconnecting the inverter to said bus, said circuit breakers includingtrip initiating means operative when actuated for causing the circuitbreakers to become nonconductive, said initiating means being actuatedin response to the detection of an abnormality in said main apparatus,the improvement comprising: switch controlling means operative inhigh-speed response to the actuation of at least one of said tripinitiating means for causing said second static switch to interrupt theflow of power through the main apparatus to said critical bus, and atthe same time for causing said first static switch to begin conductingpower from said A-C power supply to said bus, whereupon the flow of A-Cpower to said bus is virtually uninterrupted.
 1. In an electrical systemincluding main apparatus and bypass apparatus for providing A-C power toa critical bus, said bypass apparatus including a first normallynonconductive static switch connected between a supply of A-C power andsaid bus, said main apparatus including an inverter for converting D-Cpower into A-C power, a first normally conductive circuit breakerconnected between the inverter and a supply of D-C power and meanscomprising, a second normally conductive static switch in series with asecond normally conductive circuit breaker for connecting the inverterto said bus, said circuit breakers including trip initiating meansoperative when actuated for causing the circuit breakers to becomenonconductive, said initiating means being actuated in response to thedetection of an abnormality in said main apparatus, the improvementcomprising: switch controlling means operative in high-speed response tothe actuation of at least one of said trip initiating means for causingsaid second static switch to interrupt the flow of power through themain apparatus to said critical bus, and at the same time for causingsaid first static switch to begin conducting power from said A-C powersupply to said bus, whereupon the flow of A-C power to said bus isvirtually uninterrupted.
 2. The electrical system as specified in claim1 wherein said circuit breaker trip initiating means includesundervoltage sensitive means which are normally energized to a voltageabove a predetermined threshold level in response to relay meansmonitoring predetermined conditions in said main apparatus, and whereinupon the detection of an abnormality by said relay means the voltageappearing on the undervoltage sensitive devices falls below saidthreshold level.
 3. The electrical system as specified in claim 2wherein said static switch controlling means comprises means formonitoring the voltage appearing on the circuit breaker undervoltagesensitive means and for causing said second static switch to becomenonconductive and said first static switch to become conductive inhigh-speed response to the voltage on either of the undervoltagesensitive means dropping below the threshold level.
 4. The electricalsystem as specified in claim 3 wherein said main apparatus also includesmeans for monitoring fuses in the inverter and wherein said staticswitch conduction controlling means includes means for causing saidsecond static switch to become nonconductive and said first staticswitch to become conductive in high-speed response to said lastmentioned means.